Interaction of self-assembling β-sheet peptides with phospholipid monolayers: The effect of serine, threonine, glutamine and asparagine amino acid side chains

The dioleoyl phosphatidylcholine (DOPC) monolayer activities of 11 systematically altered 11 residue β-sheet tape-forming peptides were studied. Peptide–DOPC interactions were characterised by electrochemical impedance spectroscopy (EIS). An impedance model combining the constant phase element approach with dielectric relaxation in the surface layer was used to analyse the data. The facilitation of DOPC layer permeability to ions by the peptides was monitored by both EIS and the Tl(I)/Tl(Hg) and Cd(II)/Cd(Hg) faradaic reactions. It was found that peptides with side chains of serine and threonine interact with DOPC layers more strongly and in a well characterised manner compared to peptides with side chains of glutamine and asparagine. Cationic and neutral peptides containing serine and threonine penetrate the DOPC to give a maximum plateau monolayer capacitance. At higher solution concentrations of these peptides the growth of a well-defined secondary element in the impedance data indicates the segregation of secondary DOPC–peptide phases. Cationic and neutral peptides containing serine and anionic peptides containing threonine interact with the DOPC layers leading to a selective increase in the layer's permeability to Tl+ ions. Impedance measurements at higher solution concentrations of anionic peptides with serine and threonine show that these peptide modified DOPC layers associate with electrolyte ions.